The present invention relates to a plunger pump in which the interior of the bellows structure is substantially at atmospheric pressure.
U.S. Pat. No. 4,436,494 teaches a plunger pump having a bellows surrounding a plunger. The bellows is constructed of flourine plastics and the like. When the pump operates, the bellows extends and contracts within the housing of the pump, and separates the fluid to be pumped from the plunger. To offset the pressure exerted on the outer surface of the bellows by the fluid being pumped through the pump, the bellows is filled with a liquid such as oil and the like. The extension and contraction of the bellows changes the volume within the bellows, which either forces oil out of the bellows or draws oil into the bellows. Variations in the volume in the bellows is matched with a variation of volume in a liquid filled portion of an upper case portion due to the simultaneous movements of a socket constituting a part of the plunger.